Duplicating object aligned with and on faces
If this didn't change in the 2.8 (I use the 2.79), choose align to view on the editing preference

Then in edit mode choose the face you want your object to be aligned to
Hit shitf + 7 to get your view parallel to that face and shift D to duplicate the face (now extrude it and you have your cube.

If you need to extrude a face and have it's size to increase with the distance, do it with option + S (on mac)